Apparatus and method for the replacement of posterior vertebral elements

CROSS REFERENCE TO RELATED CO-PENDING APPLICATIONS

This application claims the benefit of U.S. provisional application Ser. No. 60/362,851 filed on Mar. 8^th, 2002 and entitled SPINOUS PROCESS, LAMINA, AND FACET REPLACEMENT APPARATUS which is commonly assigned and the contents of which are expressly incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an apparatus and a method for the replacement of posterior vertebral elements, and more particularly to an apparatus and a method that replaces posterior vertebral elements while preserving spinal stability and mobility.

BACKGROUND OF THE INVENTION

The human spine <b>29</b> comprises individual vertebrae <b>30</b> that interlock with each other to form a spinal column, shown in FIG. <b>1</b>A. Referring to FIGS. 1B, <b>1</b>C, and <b>1</b>D, each vertebra <b>30</b> has a cylindrical bony body (vertebral body) <b>32</b>, two pedicles <b>48</b> extending from the vertebral body <b>32</b>, a lamina <b>47</b> extending from the pedicles <b>48</b>, three winglike projections (two transverse processes <b>33</b>, <b>35</b> extending from the pedicles <b>48</b> and one spinous process <b>34</b> extending from the lamina <b>47</b>), pars interarticularis <b>36</b>, two superior facets <b>46</b> extending from the pedicles <b>48</b> and two inferior facets <b>45</b> extending from the lamina <b>47</b>. The pars interarticularis <b>36</b> connects the superior <b>46</b> and inferior <b>45</b> facets on either side of the spinous process <b>34</b>. The bodies of the vertebrae <b>32</b> are stacked one on top of the other and form the strong but flexible spinal column. The spinous process <b>34</b>, lamina <b>47</b>, pars interarticularis <b>36</b>, superior facets <b>46</b>, inferior facets <b>45</b>, transverse processes <b>33</b>, and pedicles <b>48</b> are positioned so that the space they enclose forms a tube, i.e., the spinal canal <b>37</b>. The spinal canal <b>37</b> houses and protects the spinal cord and other neural elements. A fluid filled protective membrane, the dura <b>38</b>, covers the contents of the spinal canal. The spinal column is flexible enough to allow the body to twist and bend, but sturdy enough to support and protect the spinal cord and the other neural elements.

The vertebrae <b>30</b> are separated and cushioned by thin pads of tough, resilient fiber known as inter-vertebral discs <b>40</b>. Inter-vertebral discs <b>40</b> provide flexibility to the spine and act as shock absorbers during activity. There is a small opening (foramen) <b>42</b> between each vertebra <b>30</b>, through which nerves <b>44</b> pass and go to different body parts. When the vertebrae are properly aligned the nerves <b>44</b> pass through without a problem. However, when the vertebrae are misaligned or a constriction <b>45</b> is formed in the spinal canal, the nerves get compressed <b>44</b><i>a </i>and may cause back pain, leg pain or other neurological disorders. Disorders of the spine that may cause misalignment of the vertebrae or constriction of the spinal canal include spinal injuries, infections, tumor formation, herniation of the inter-vertebral discs (i.e., slippage or protrusion), arthritic disorders, and scoliosis. In these pathologic circumstances, surgery may be tried to either decompress the neural elements and/or fuse adjacent vertebral segments. Decompression may involve laminectomy, discectomy, or corpectomy. Laminectomy involves the removal of part of the lamina <b>47</b>, i.e., the bony roof of the spinal canal. Discectomy involves removal of the inter-vertebral discs <b>40</b>. Corpectomy involves removal of the vertebral body <b>32</b> as well as the adjacent disc spaces <b>40</b>. Laminectomy and corpectomy result in central exposure of the dura <b>38</b> and its contents. An exposed dura <b>38</b> puts the neural elements and spinal cord at risk from direct mechanical injury or scarring from overlying soft tissues. Scarring is considered a major cause for failed back syndrome in which patients continue to have back and leg pain after spinal surgery. Current methods to decrease the risk of developing this syndrome include covering the dura with fat harvested from the patient's subcutaneous tissues or using a synthetic material. However, no material as yet has been used that completely or significantly prevents scarring of the dura and nerve roots after spine surgery in humans.

Furthermore, laminectomy predisposes the patient to instability through the facet joints and may lead to post-laminectomy kyphosis (abnormal forward curvature of the spine), pain, and neurological dysfunction. Therefore the surgeon needs to stabilize the spine after laminectomy procedures and after corpectomy. One spine stabilization method is fusion. Fusion involves the fixation of two or more vertebrae. Fusion works well because it stops pain due to movement of the intervertebral discs <b>40</b> or facets <b>45</b>, <b>46</b>, immobilizes the spine, and prevents instability and or deformity of the spine after laminectomy or corpectomy. However, spinal fusion limits spinal mobility. Maintaining spinal mobility may be preferred over fusion in some cases to allow more flexibility of the spine and to decrease the risk of junction problems above and below the level of the fixation due to increased stress.

An arthritic facet joint may also cause back pain. Since the majority of the motion along the spine occurs at the facet joints, fusing the diseased facet would often relieve pain but again at a high cost of fusing across at least one spinal segment thus preventing motion and effectively increasing stresses at the adjacent facet joints. Increased stresses predispose facet joints to accelerated arthritis, pain, and instability requiring additional surgery to fuse these levels. This cyclic process results in an overall decreased mobility of the spine. Therefore, it is an attractive alternative to attempt to replace the diseased facet without resorting to fusion, thus avoiding significant limitation in mobility of the spine. The obvious solution would be to replace the opposing surfaces of each facet to preserve motion between the surfaces. Any efforts to replace the facets at their natural location necessitate destroying the facet capsule and risks producing an unstable joint. It would be desirable to achieve spine stabilization that preserves mobility, protects the contents of the spinal canal, does not cause tissue scarring, decreases pain in the facet joints, and does not always destroy the facet capsule.

SUMMARY OF THE INVENTION

In general, in one aspect, the invention features an orthopedic implantable device articulately connecting a first spinal vertebra to an adjacent second spinal vertebra. Each spinal vertebra includes a vertebral body, a pair of pedicles extending posteriorly from the vertebral body, a lamina extending from the pedicles, a pair of superior facets extending from the pedicles, a pair of inferior facets extending from the lamina, a pair of pars interarticularis connecting the superior and inferior facets, a spinous process extending from the lamina and a pair of transverse processes extending from the pedicles. The orthopedic implantable device includes a first component adapted to be attached to a posterior location of the first vertebra and a second component adapted to be attached to a posterior location of the second vertebra. The first component is articulately connected to the second component.

Implementations of this aspect of the invention may include one or more of the following features. The first component may be articulately connected to the second component along a posterior midline of the first and second vertebrae and/or along an axis medial to the facets of the vertebrae. The first component may comprise a body and at least one male articulation member attached to the first component body and the second component may comprise a body and at least one female articulation member attached to the second component body and the first component may be articulately connected to the second component by engaging the at least one male articulation member to the at least one female articulation member. The at least one male articulation member may comprise a hook and the at least one female articulation member may comprise a loop. The first component body may further comprise at least one female articulation member and the second component body may further comprise at least one male articulation member. The posterior locations of the first and second vertebrae are selected from a group including a pedicle, transverse processes, facets, lamina, pars interarticularis, and vertebral body. The body of the first component may be attached to first and second pedicles of the first vertebra and the body of the second component may be attached to first and second pedicles of the second vertebra, respectively. The first and second components may be attached to the first and second vertebrae, respectively, via screws, wires, or hooks. The first component may be articulately connected to the second component via a hinge. The first and second components may have adjustable width and the width may be adjusted between 20 and 80 millimeters.

In general, in another aspect, the invention features an orthopedic implantable device articulately connecting a plurality of individual vertebrae. The vertebrae are adjacent to each other and form a segment of a spinal column. The orthopedic device includes a plurality of individual components, and each component is adapted to be attached to a posterior location of an individual vertebra. Each of the individual components is articulately connected to an adjacent inferior and an adjacent superior component along a posterior midline of the spinal column or any axis medial to the facets.

Implementations of this aspect of the invention may include one or more of the following features. Each component may comprise a body, at least one female articulation member formed within the body, and at least one male articulation member attached to the body. Each of the individual components may be articulately connected to an adjacent inferior component by engaging the at least one male articulation member to the at least one female articulation member of the adjacent inferior component, and each of the individual components may be articulately connected to an adjacent superior component by engaging the at least one female articulation member to the at least one male articulation member of the superior component.

In general, in another aspect, the invention features an orthopedic implantable component adapted to replace a posterior element of a vertebra. The vertebra comprises a vertebral body, a pair of pedicles extending posteriorly from the vertebral body, a lamina extending from the pedicles, a pair of superior facets extending from the pedicles, a pair of inferior facets extending from the lamina, a pair of pars interarticularis connecting the superior and inferior facets, a spinous process extending from the lamina and a pair of transverse processes extending from the pedicles. The orthopedic component includes a body adapted to be attached to a posterior location of the vertebra, a male articulation member extending from the body, and a female articulation member formed within the body.

Implementations of this aspect of the invention may include one or more of the following features. The body may be configured to replace the lamina, the spinous process, pars interarticularis, and the superior and inferior facets of the vertebra. The body may be attached to the pedicles of the vertebra via screws threaded through apertures formed in the body. The screws may be made of stainless steel, titanium, gold, silver, alloys thereof, plastic, absorbable or biodegradable material. The orthopedic implantable component may be made of metal, plastic, ceramic, bone, polymers, composites, absorbable material, biodegradable material, and combinations thereof. The male articulation member may be a hook and the female articulation member may be a bar connecting opposite sides of a cavity formed within a bottom surface of the body. The cavity may be arranged along a midline of the body or along any axis medial to the facets.

In general, in another aspect, the invention features a spine stabilization method articulately connecting a first vertebra to a second vertebra including the following steps. First providing a first component and attaching the first component to a posterior location of the first vertebra. Next, providing a second component and attaching the second component to a posterior location of the second vertebra. Finally, articulately connecting the first component to the second component.

In general, in another aspect, the invention features a spine stabilization method connecting a first vertebra to a second vertebra including the following steps. First attaching first and second screws to first and second locations, respectively, of the first vertebra. Next attaching third and fourth screws to first and second locations, respectively, of the second vertebra. Next providing first and second components, the first and second components comprising a body and being articulately connected to each other along a midline of the bodies. Next attaching the body of the first component to the first and second locations of the first vertebra via the first and second screws, respectively. Next, attaching the body of the second component to the first and second locations of the second vertebra via the third and fourth screws, respectively. Finally, tightening of all said screws.

Implementations of this aspect of the invention may include before attaching the bodies of the first and second components adjusting the width of the bodies of the first and second components.

Among the advantages of this invention may be one or more of the following. The implantable spinal stabilization device stabilizes the spine, while allowing the patient to retain spinal flexibility by preserving motion between adjacent vertebras. The spinal stabilization device may be used for the treatment of a multitude of spinal disorders including facet arthritis and spinal stenosis. The device covers and protects the posterior central neural elements after laminectomy, and replaces diseased or injured facet joints without fusing across vertebral segments. The invention can be used to replace the spinous process, laminas, and facets individually or in combination at each vertebra. The implantable device has a compact structure and low profile. By virtue of transferring the motion between adjacent vertebras from the diseased or iatrogenically injured facets laterally towards the midline, the spine is stabilized but also allows motion between the vertebrae. This is a desirable feature when compared to fusion in selected cases where preserving a patient's spinal mobility is preferred.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and description below. Other features, objects and advantages of the invention will be apparent from the following description of the preferred embodiments, the drawings and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the figures, wherein like numerals represent like parts throughout the several views:

FIG. 1A is a side view of the human spinal column;

FIG. 1B is an enlarged view of area A of FIG. 1A;

FIG. 1C is an axial cross-sectional view of a lumbar vertebra;

FIG. 1D is a perspective view of a lumbar vertebra;

FIG. 2 is a schematic posterior view of an implantable spine stabilization device according to this invention;

FIG. 3 is a posterior view of a spine stabilization component of the implantable spine stabilization device of FIG. 3;

FIG. 4 is a perspective view of a lumbar vertebra with resected spinous process, lamina, and facet joints and the stabilization component of FIG. 3 attached to its pedicles;

FIG. 4A is a cross-sectional view of FIG. 3 along AA′ plane;

FIG. 5 is a perspective view of the spine stabilization component of FIG. 3;

FIG. 5A is a cross-sectional view of FIG. 3 along BB′ plane;

FIG. 6 is a cross-sectional side view of the spine stabilization device of FIG. 2 along midline <b>102</b>;

FIG. 7 is a posterior view of a spine stabilization component without a tail segment; and

FIG. 8 is a flow diagram depicting the method of applying the implantable spine stabilization device of this invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2, an implantable spine stabilization device <b>100</b> connects vertebra <b>92</b> to adjacent vertebra <b>94</b> and vertebra <b>94</b> to adjacent vertebra <b>96</b>. The spine stabilization device <b>100</b> includes modular components <b>110</b>, <b>120</b>, and <b>130</b>. Modular components <b>110</b>, <b>120</b>, and <b>130</b> have circular ends <b>110</b><i>a </i>and <b>110</b><i>b</i>, <b>120</b><i>a </i>and <b>120</b><i>b</i>, <b>130</b><i>a </i>and <b>130</b><i>b</i>, respectively, that attach to pedicles <b>92</b>A, <b>92</b>B, <b>94</b>A, <b>94</b>B, <b>96</b>A, and <b>96</b>B of vertebra <b>92</b>, <b>94</b> and <b>96</b>, respectively, via pedicle screws <b>111</b><i>a</i>, <b>111</b><i>b</i>, <b>121</b><i>a</i>, <b>121</b><i>b</i>, <b>131</b><i>a</i>, and <b>131</b><i>b</i>, respectively. Modular components <b>110</b>, <b>120</b>, and <b>130</b> replace the resected laminas, pars interarticularis, facets and spinous processes of the vertebra <b>92</b>, <b>94</b>, and <b>96</b>, respectively. Modular component <b>110</b> is articulately connected to component <b>120</b> along the midline <b>102</b> of the device <b>100</b> and the corresponding vertebrae <b>92</b> and <b>94</b>, shown in FIG. <b>6</b>. Similarly modular component <b>120</b> is articulately connected to component <b>130</b>. Additional modular components may be added to extend the spine stabilization device <b>100</b> in either caudad <b>272</b> or cephalad <b>270</b> directions. The modular structure of the spine stabilization device <b>100</b> allows a surgeon to replace laminas, facets, pars interarticularis, and spinous processes over any distance and orientation along the entire spine <b>29</b>.

Referring to FIG. 3, modular component <b>110</b> comprises a lamina <b>114</b>, two circular ends <b>110</b><i>a</i>, <b>110</b><i>b </i>extending laterally from opposite sides of the lamina <b>114</b>, a tail segment <b>118</b> extending from the lower portion of the lamina <b>114</b>, and a spinous process <b>116</b> protruding posteriorly from the top surface of the lamina <b>114</b>. The lamina <b>114</b> has a width <b>81</b> and a length 82 that depend upon the distance between the pedicles <b>92</b>A and <b>92</b>B and the length of the vertebra <b>92</b>, respectively. The length 83 of the tail segment <b>118</b> depends upon the intervertebral distances. In one example, the width 81 is in the range between 20 millimeters and 80 millimeters, length 82 is in the range of 10 millimeters and 80 millimeters, length 83 is in the range of 4 millimeters and 60 millimeters and height 84 is in the range of 4 millimeters and 30 millimeters. Width 81, length 82, length 83 and height 84 have different values for the different type of vertebrae, including lumbar, thoracic, sacral and cervical.

Referring to FIG. <b>3</b> and FIG. 5, pedicle screw <b>111</b><i>b </i>comprises a body portion <b>140</b>, a first head portion <b>142</b>, a second head portion <b>144</b>, and a head <b>146</b>. The body portion <b>140</b> of the pedicle screw <b>111</b><i>b </i>has helical threads on its exterior surface and screws into the vertebral body <b>32</b> through the pedicle <b>92</b>B. A hexagonal screwdriver (not shown) is inserted into a slot <b>148</b> formed on the head <b>146</b> of the pedicle screw <b>111</b><i>b </i>and is used to drive the screw <b>111</b><i>b </i>into the vertebral body <b>32</b>. The first head portion <b>142</b> is directly above the body portion <b>140</b> and has a smooth or serrated outer surface <b>143</b> for receiving the circular end <b>110</b><i>b </i>of modular component <b>110</b>. End <b>110</b><i>b </i>has an aperture <b>152</b><i>b </i>that allows end <b>110</b><i>b </i>to slide over the pedicle screw <b>111</b><i>b</i>. The second head portion <b>144</b> has a threaded outer surface for receiving locking nut <b>112</b><i>b</i>. Locking nut <b>112</b><i>b </i>slides over the head <b>146</b> of the pedicle screw <b>111</b><i>b </i>and screws around the threaded outer surface of the second head portion <b>144</b>, thus securely attaching the circular end <b>110</b><i>b </i>to pedicle screw <b>111</b><i>b</i>. In one example, pedicle screw <b>111</b><i>a </i>has a length 220 of 57 millimeters and a diameter <b>222</b> of 6.5 millimeters.

Referring to FIG. <b>4</b> and FIG. 4A, the cross-section of lamina <b>114</b> along AA′ has a U-shape and the roof <b>114</b><i>c </i>of the lamina (top of U-shape) is elevated above the spinal canal <b>37</b>. The sides <b>114</b><i>a </i>and <b>114</b><i>b </i>of the lamina <b>114</b> run first at a gentle slope downwards about 5 degrees and then drop more sharply at about 80 degrees to get to the pedicles <b>92</b>A and <b>92</b>B, respectively. The U-shape form of the lamina <b>114</b> provides space between the spine stabilization device <b>100</b> and the spinal canal <b>37</b> and is also designed to clear the facets <b>46</b> laterally, in case they were not previously resected. This arrangement covers the central spinal canal and protects the neural elements from scar tissue formation or mechanical damage. The lamina <b>114</b> has a flared lower portion that extends into the tail segment <b>118</b>.

In the embodiment of FIG. 4A the width <b>81</b> of the lamina <b>114</b> is extended or contracted via mechanism <b>90</b>. In this embodiment the lamina <b>114</b> comprises a first segment <b>92</b> and a second segment <b>94</b>. Segment <b>94</b> is allowed to slide in the lateral direction <b>86</b> and can rotate around the axis <b>87</b>. The lateral motion of segment <b>94</b> allows the adaptation of the modular component <b>110</b> to vertebrae with various pedicle distances. The rotation of segment <b>94</b> around the axis <b>87</b> allows accurate positioning of the circular end <b>110</b><i>b </i>over the pedicle screw <b>111</b><i>b </i>and accommodates pedicles that are not perfectly aligned in the cephalo-caudad direction. Segments <b>94</b> and <b>92</b> have overlapping elongated slots <b>184</b>A and <b>184</b>B, respectively, extending through the thickness of the corresponding segment. A housing <b>182</b> slides over the overlapping segments <b>92</b> and <b>94</b>. Housing <b>182</b> has an elongated slot <b>186</b> that runs through the thickness of the housing <b>182</b> and is aligned with the elongated slots <b>184</b>A and <b>184</b>B. The position of the overlapping segments <b>92</b> and <b>94</b> and the housing <b>182</b> is secured via a screw <b>188</b> that is threaded through the elongated slots <b>184</b>A, <b>184</b>B, and <b>186</b>. In one example, the width 81 of the lamina <b>114</b> is 40 millimeters and it can be increased or decreased up to 8 millimeters via the two sliding mechanisms <b>90</b>. Circular ends <b>110</b><i>a</i>, <b>110</b><i>b </i>have apertures <b>152</b><i>a</i>, <b>152</b><i>b</i>, respectively. Apertures <b>152</b><i>a </i>and <b>152</b><i>b </i>have serrated inner surfaces for receiving a pedicle screw with matching longitudinal serrations <b>143</b>, shown in FIG. <b>5</b>. The top and/or bottom surfaces of circular ends <b>110</b><i>a</i>, <b>110</b><i>b </i>have radial extending grooves <b>88</b> that match the grooves <b>89</b> of the locking nuts <b>112</b><i>a</i>, <b>112</b><i>b. </i>

Referring to FIG. <b>5</b> and FIG. 5A, the posteriorly protruding spinous process <b>116</b> includes a cavity <b>115</b> formed in the bottom surface of the lamina <b>114</b> within the spinous process <b>116</b>. Inside the cavity <b>115</b> there is a horizontally extending bar <b>117</b> attached to opposite cavity walls <b>115</b><i>a</i>, <b>115</b><i>b</i>. Referring to FIG. 6, the end of the tail segment <b>118</b> of modular component <b>110</b> forms a hook <b>119</b>. Hook <b>119</b> engages around the horizontal bar <b>117</b> of the adjacent modular component <b>120</b> and forms an articulated connection between the two modular components <b>110</b> and <b>120</b>. The cavity <b>115</b> is contoured to allow smooth gliding of the outer surface <b>118</b><i>a </i>of the tail segment <b>118</b> around the horizontal bar <b>117</b>.

Referring to FIG. 7, a modular component <b>140</b> without the tail segment <b>118</b> is implanted to the pedicles on the vertebra that is below but adjacent to the lowest (in the caudad direction <b>272</b>) level that underwent either a laminectomy or facetectomy. This vertebra will still have its natural spinous process and ligamentous attachment to the next lower vertebra. This vertebral level will therefore provide stability to the end of the stabilization assembly <b>100</b> since this vertebral level will have preserved facets and ligamentous attachments.

Referring to FIG. 8, a method <b>400</b> of using the spine stabilization device <b>100</b> comprises the following steps. Opening an incision in the patient's back, and exposing first and second vertebrae, the vertebra that is immediately above but adjacent to the first vertebra (cephalad direction), and the vertebra that is immediately below but adjacent to the second vertebra (caudad direction) (<b>405</b>). Performing laminectomy and/or facetectomy posteriorly of the first and second vertebrae (<b>410</b>). Placing pedicle screws within the pedicles of the first and second vertebra, the vertebra immediately above the first vertebra, and the vertebra immediately below the second vertebra (<b>420</b>). Engaging a first modular component to a second modular component. In one example, the modular components are as shown in FIG. <b>3</b>. Placing the apertures of the two circular ends <b>110</b><i>a</i>, <b>110</b><i>b </i>of the first modular component over the two contralateral pedicle screws on the first vertebra, and adjusting the length and orientation of the two end segments <b>94</b> of the lamina (<b>430</b>). Placing the apertures of the two circular ends <b>110</b><i>a</i>, <b>110</b><i>b</i>, of the second modular component over the two contralateral pedicle screws on the second vertebra, and adjusting the length and orientation of the two end segments <b>94</b> of the lamina (<b>440</b>). Engaging a third modular component without a tail segment, as shown in FIG. 7, to the tail of the second modular component, placing the apertures of the two circular ends <b>110</b><i>a</i>, <b>11</b><i>ab</i>, of the third modular component over two contralateral pedicle screws on the vertebra that is immediately below the second vertebra, and adjusting the length and orientation of the two end segments <b>94</b> of the lamina (<b>450</b>). Engaging a fourth modular component to the first modular component, placing the apertures of the two circular ends <b>110</b><i>a</i>, <b>110</b><i>b</i>, of this fourth modular component over the two contralateral pedicle screws on the vertebra that is immediately above the first vertebra, and adjusting the length and orientation of the two end segments <b>94</b> of the lamina (<b>455</b>). Tightening of the nuts over the pedicle screws down on the circular ends (<b>460</b>) and closing of the incision in the patient's back (<b>470</b>).

Other embodiments are within the scope of the following claims. For example, the articulation mechanism between the modular components may be a hinge. There may be more than one articulation mechanisms medial or lateral to the medial line <b>102</b> on a given vertebra, and/or medial to both the natural facet joints. The ends of the modular components may be secured to pedicle screws via connectors. The ends of the modular components may be attached to the vertebrae via hooks. Other locations where screws, wires, or hooks may be anchored for attaching the stabilization device of this invention include the transverse processes <b>33</b>, <b>35</b>, the vertebral body <b>32</b>, and the lamina <b>47</b>. The modular components may be solid without adjustable ends. Modular components <b>110</b>,<b>120</b>,<b>130</b> and <b>140</b> may be manufactured from a variety of materials including among others stainless steel, titanium, nickel, composites, ceramics, plastic, bone, bioabsorbable material or combination thereof. Pedicle screws may be manufactured from a variety of materials including among others stainless steel, titanium, gold, silver ceramics, plastic, bioabsorbable material, or alloys thereof.

Several embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.